Increasing diversity of nitrides in iron meteorites: discovery and formation conditions of siderazot in Sikhote-Alin IIAB

Abstract

Nitrides are a small group of minerals whose formation is primarily associated with reducing conditions. Roaldite (Fe4N or γ′-Fe4N) and siderazot (Fe3N1.33 or ɛ-Fe3N) are chemically similar minerals, yet they are not commonly found together. While roaldite is typical of iron meteorites and has not been detected on Earth, siderazot is predominantly found in terrestrial rocks and has never been found in iron meteorites. We studied the Sikhote-Alin iron meteorite using optical and scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), wavelength-dispersive X-ray spectroscopy (WDS), and electron backscatter diffraction (EBSD) and demonstrated the presence of siderazot. We also demonstrated that roaldite is a typical accessory mineral of the Sikhote-Alin iron, and that nitride detection can be easily accomplished using routine nitrogen mapping. In the Sikhote-Alin iron, both roaldite and siderazot contain very low amounts of nickel (< 5 wt. % Ni), which is typical of the host kamacite (α-Fe). It is noteworthy that roaldite and siderazot in the Sikhote-Alin iron differ only slightly in nitrogen content (∼6 and 8 wt. % N), which makes it difficult to identify them without the information on the crystal structure. However, roaldite and siderazot can be reliably identified based on EBSD data, given the significant differences in their unit cell parameters and crystal structures (space groups Pm-3m and P6322, respectively). We propose that siderazot formation is associated with impacts on primary roaldite and marks high-pressure transformations of meteoritic material. Searching for siderazot (and other iron nitrides like Fe2N or Fe16N2) in other iron meteorites (especially those in which roaldite has previously been detected) may help to establish the mechanism of their formation and transformation.